Clearly, it is necessary to ensure that components, and in particular critical components within safety and reliability driven technologies such as aircraft engines, meet necessary performance standards. Particular components within an aircraft engine which must be tested in order to determine their life expectancy within the operational environment of an engine are the turbine discs and in particular the high temperature turbine discs within a gas turbine engine. It will be understood that if such turbine discs failed in use there may be catastrophic consequences.
Unfortunately, modern turbine discs are of relatively large size with a large bore section. In such circumstances, during transient operation of an engine incorporating such turbine discs, the speed and attached blade weight to these discs can cause high (circumferential hoop) stressing to the disc upon which the blades are secured. In such circumstances it is necessary to test turbine discs and in particular the materials from which such discs are formed in order to determine expected life in an engine.
Previously, testing of turbine discs has been achieved by simple spin testing of an actual or slightly modified real engine turbine disc component. However, such an approach is undermined by the nature of large bore discs in that in operation, that is to say within an engine, the bore section takes time to heat up as a result of external heating within an operating engine. During this time the bore surface heats first whilst the centre of the bore remains relatively cool. This thermal gradient imparts a compressive axial stress on the bore that may result in erroneous test results. Generally, the compressive axial stress when added to high hoop stress due to rotation leads to a combined (Von-Mises) stress which is often far higher than the simple circumferential hoop stresses tested. In such circumstances, it is believed that the stress level tested using existing hoop stress testing techniques will give unrepresentative detected or projected operational lives compared to testing that covers the actual combination of compressive axial as well as tensile hoop stresses.